I don't know--is it a holdover from the Sixties protest era?
My dad isn't an active musician but he loves music (folk music, all kinds of music). He did graduate field work in Michigan 30 years ago and, if he had a beard, he could feel the bugs coming before they could bite him, but I don't think that's why the majority of folkies have beards. I grew up in Quaker meeting with plenty of bearded men but most of them were Sixties holdovers. Shall we examine photographic documentation--when did folkies start growing beards?